1. Field of the Invention
This invention relates to improvements in vertical continuous casting or the casting technique of obtaining cast pieces by drawing a strand formed in a water-cooled mold downward without incurvating, and then cutting the strand.
2. State of the Art
Nowadays, the continuous casting process holds an important position in a steel casting. However, the horizontal continuous casting process is somewhat problematical from a technical point of view. For such reasons it has only been tried in part. Accordingly, the curved and vertical continuous casting processes are overwhelmingly used today. The vertical continuous casting is disadvantageous because it requires equipment which has a high cost. Accordingly, the curved continuous casting process is predominately used. In this process a strand is incurvated (a water-cooled mold is structured so as to provide the curved strand) to extend horizontally, thereby keeping a height of the casting equipment within a certain limit. The strand extending horizontally is rectified to be straight with pinch rolls and cut with a frame cutter into cast pieces.
However, such a process comprising curved casting and horizontal rectification is not necessarily preferable for some special kinds of steel with respect to producing castings of sufficient quality. For example, the rectification operation often causes cracks on the strands of valve steel, high-speed steel and other steels. Accordingly, this kind of steel will have to be casted by the vertical continuous casting wherein a strand is formed without curvature in the water-cooled mold, drawn straightly downward and cut to cast pieces.
In the vertical continuous casting process, light reduction should be applied to the strand coming out of the water-cooled mold at a position immediately before an unsolidified portion disappears in the interior of the strand, whereby contraction of the strand due to solidification is compensated, and thus, formation of cavities at the center of the strand may be prevented, and center segregation is lessened as well. In this way cast pieces of better quality can be obtained.
However, in the case of some steels for which the application of light reduction is effective, different kinds of steels are usually produced in relatively small quantities. Therefore, the sectional size and the pertinent drawing speed of a strand varies. For such reasons, the optimal position at which the light reduction is to be applied, namely, the point at which the unsolidified portion of the strand disappears can vary from one strand to another.